1. Technical Field
The present invention relates generally to power control in code division multiple access (CDMA) cellular systems. More particularly, the present invention relates to controlling forward link power in CDMA cellular systems.
2. Discussion
With the growing mobility of today""s society, cellular phones have become a part of daily life. To support this growth, standards have been developed such as IS-95 to provide compatibility among base stations and mobile subscribers. Third generation (3G) systems such as CDMA 2000 and W-CDMA are an evolutionary outgrowth of IS-95 systems, and attempt to offer a seamless migration path that economically supports upgrades to 3G features and services. These features and services are typically within existing spectrum allocations for both cellular and PCS operators.
As part of the growth in cellular technology, a transition from TDMA to CDMA has taken place. After digitizing data, CDMA spreads the data out over the entire available bandwidth as opposed to narrow bandwidth transmissions associated with TDMA. Multiple calls occupy a common channel, with each assigned a unique sequence code. The goal of spread spectrum is to provide a substantial increase in bandwidth of an information-bearing signal, far beyond that needed for basic communication. The bandwidth increase, while not necessary for communication, can mitigate the harmful effects of interferencexe2x80x94either deliberate (i.e. a military jammer), or inadvertent (i.e. co-channel users). In fact, interference mitigation is a well-known property of all spread spectrum systems. In the past, however, CDMA (and spread spectrum in general) was dismissed as unworkable in the mobile radio environment because of what was called the xe2x80x9cnear-far problem.xe2x80x9d This is because it was always assumed that all base stations transmitted constant power. In the mobile radio environment, however, some users may be located near the base station and others may be located far away. The propagation path loss difference between those extreme users can be many tens of dB. The result is that in order to accommodate the farthest users, spectral efficiency is sacrificed for the nearest users.
In response to the above near-far problem, xe2x80x9cpower controlxe2x80x9d has evolved in order to allow the transmitters to control transmitted power in such a way that the received powers from all users are roughly equal. This allows the benefits of spread spectrum techniques to be realized. If the received power is controlled, then the mobile subscribers can occupy the same spectrum, and the above described benefits of interference averaging accrue.
Under conventional power control techniques, a local transmitter can either directly adjust its own power based on a received signal (open loop) or indirectly adjust its own power based on the signal transmitted to a remote receiver by the local transmitter (closed loop). In the closed loop scenario, the remote receiver is also a transmitter and returns a power control command based on either a frame error rate (FERxe2x80x94outer loop), or a signal to interference ratio (SIR--inner loop) of the signal transmitted by the local transmitter.
In the outer loop scenario, the remote transmitter (e.g. mobile subscriber) determines the power control command based on the amount of error in each received frame. Upon receiving the power control command from the remote transmitter, the local transmitter (e.g. base station) can define a physical layer response to the request.
Under IS-95 systems, the physical layer will perform modulation, coding, power control, and synchronization functions. Upper layers (i.e. the data link layer and the network layer) perform functions such as traffic, pilot, paging, access, messaging, mobility and radio resources management.
A difficulty associated with outer loop power control, however, relates to the fact that the typical frame size of a CDMA 2000 system traffic channel is 20 ms, whereas a power control command is transmitted every 1.25 ms. Inner loop power control was therefore developed to address the problem of power control during frame acquisition. Inner loop power control allows the remote transmitter to determine the power control command based on an SIR measurement. Thus, upon receiving the power control command, the base station can continue to define a physical layer response to the request. If perfect signal orthogonality can be realized, then the remote transmitter can identify its own signal very well. However, due to the multipath effect on air transmissions, orthogonality will often be destroyed on the remote transmitter side. In such cases, the remote transmitter cannot clearly identify its own signal as opposed to interference. Since each 20 ms frame includes 16 power commands, a high rate remote transmitter could send out consecutive power up commands for the entire frame when the transmitted power level is actually sufficient. If the base station follows the conventional rule of increasing the transmitting power level based on the power command, the cumulative power adjustment could cause serious problems for the entire cell. It is therefore highly desirable to provide a method for controlling a forward link power level in a CDMA cellular system that does not fall subject to multipath affects.
The above and other objectives are provided by a base station controller and method for controlling a forward link power level in a CDMA cellular system. The method includes the steps of defining an adjustment threshold for the forward link power level, and receiving an inner-loop power control command from a remote transmitter. The power control command defines a request for an incremental adjustment in the forward link power level. The method further provides for defining the forward link power level based on the adjustment threshold and the request.
The present invention also provides a method for defining a forward link power level in a CDMA cellular system based on an adjustment threshold and an incremented adjustment request. The method includes the step of tracking a cumulative power adjustment of the forward link power level. The cumulative power adjustment is compared to the adjustment threshold, and a physical layer response to the request is defined based on the comparison.
In another aspect of the invention, a base station controller controls a forward link power level in a CDMA cellular system. The control system includes a microcontroller unit for defining an adjustment threshold for the forward link power level, and a signal processing system for receiving an inner loop power control command from a remote transmitter. The power control command defines a request for an incremental adjustment in the forward link power. The base station controller further includes a power control system for defining the forward link power level based on the adjustment threshold and the request.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.